What is a Digital to Analog Converter (DAC)? Choosing the Right DAC for Your Project

What is a DAC?

Digital-to Analog-Converters, or DACs, are a ubiquitous feature of modern electronics. These converters are a necessary component in any application that involves taking in digital information and changing it to a form that analog devices can interact with. Simply put, DACs are a necessary step in translating the ones and zeroes in your computer to video, audio, and other sensory data that you can perceive. In some instances, they can also translate digital instructions to physical objects, allowing users to control mechanical devices digitally.

Digital to Analog Converter Uses Of these mechanical devices, the most common application is audio reproduction. Any audio player with a digital source will need to convert bits and bytes into an electrical output that can be used to drive a speaker. These devices can include:

- CDs

- DVDs

- Cell phones

- Mp3 players

- Computer sound cards

We also commonly see DACs at work in video, as nearly all modern video displays and formats use a digital method of storing the image data.

Other applications for DACs include:

- Certain ADC (analog-to-digital converter) architectures that incorporate a DAC as a control device – comparing the ADC output data to reconverted DAC data as a way of checking and refining their output

- Calibration in test and measurement systems

- Motor controls

Selection Criteria for DAC: We can evaluate DACs based on several criteria:

Resolution: Typically, we express resolution in bits, which represent the base two logarithms of possible output levels it can reproduce. For example, an 8-bit DAC can produce 2⁸ (256) levels. This figure is the parameter that establishes color depth in video applications and audio bit depth in audio applications. Essentially, the resolution establishes the least significant bit (LSB) which, in DACs, refers to the smallest increment change of the DAC output in response to a digital input.

Speed: We use the term maximum sample rate to refer to the maximum speed that the system can maintain while continuing to create the correct output.

Monotonicity: Monotonicity defines the DAC’s ability to maintain output that follows the direction of the input. That is, a DAC shouldn’t experience a spike before trending downward when the input decreases.

Dynamic range: We express dynamic range in decibels. This range provides the difference between the maximum and minimum output signal the system can achieve.

Digital to Analog Converter Architectures: The two most common DAC architectures are called “weighted resistor” and “R-2R.”  Weighted resistor DACs use a bank of switched resistors. Here’s a quick guide to how they operate, using the voltage domain as an example:

1. The DAC consists of a logic circuit connected to resistors, each of which corresponds to a particular bit (for instance, an 8-bit DAC would have eight resistors, corresponding to 2⁰ to 2⁷). The resistors are of values weighted in a binary format (each resistor is a power of two stronger than the one before it).

2. The DAC’s logic circuit receives all the digital input bits simultaneously, receiving either a zero or a one for each switch/resistor from the binary input.

3. When the logic circuit receives a zero, the switch connects to ground.

4. When receiving a one, the logic circuit connects the switch to the resistor.

5. At the output, a summing amplifier combines the voltages to form the output signal, converting the digital voltages to a summed analog output.

6. R-2R devices feature a repeating structure of resistors with values R and 2R. This design makes it easier to produce value-matched resistors and improves precision.

Finally, you might sometimes see devices labeled as “segmented” DACs. In a segmented DAC, the component employs multiple architecture types – one to handle the most significant bits and the other to handle the least significant bits. Manufacturers designed segmented DACs to achieve peak performance from the device, so while these components tend to perform well, they may also be more expensive.

Choosing the Best DAC for Your Project

Most information that’s available about “choosing the right DAC” points you toward ready-to-use audio devices. These DACs go between the source of a digital audio file and the speakers that reproduce it to improve audio quality. That makes sense, since it’s the most common application for DAC technology.

For those building their own devices, however, it can be harder to find a way to break things down. Still, you won’t have to build or integrate your own. Most available DAC components are integrated circuits, since each component making up the DAC needs to be precisely matched. Discrete DACs are widely available.

When choosing a DAC for whatever your next project may be, start by defining the needs. Ask yourself the following questions:

1. What will you be using the DAC to decode? Audio, video, or something else?

2. How much data will it need to process and is speed or resolution more critical for this application?

3. How many output channels will you need?

Answering these questions before you begin your search will narrow down the field considerably.

If you’re working with video, speed is likely going to be more important than resolution, especially if you’re building a small or portable device. Because of the large amount of data required for video, speed is critical. If the display isn’t large, you can further sacrifice on resolution.

For audio, you’ll need to make the opposite considerations. Your DAC will require comparatively little data, even for lossless audio files (.FLAC). For these applications, opt for a device with good resolution.

You may also want to consider the DAC architecture when making your selection. R-2R DAC devices offer a greater amount of precision, and segmented DACs are typically performance-built. For more demanding applications, an R-2R DAC may be the best choice, or at least worth your consideration.